4,322 research outputs found
Entanglement oscillations in non-Markovian quantum channels
We study the non-Markovian dynamics of a two-mode bosonic system interacting
with two uncorrelated thermal bosonic reservoirs. We present the solution to
the exact microscopic Master equation in terms of the quantum characteristic
function and study in details the dynamics of entanglement for bipartite
Gaussian states. In particular, we analyze the effects of short-time
system-reservoir correlations on the separability thresholds and show that the
relevant parameter is the reservoir spectral density. If the frequencies of the
involved modes are within the reservoir spectral density entanglement persists
for a longer time than in a Markovian channel. On the other hand, when the
reservoir spectrum is out of resonance short-time correlations lead to a faster
decoherence and to the appearance of entanglement oscillations.Comment: 5 pages, 2 figures, published versio
Evaluating Cartogram Effectiveness
Cartograms are maps in which areas of geographic regions (countries, states)
appear in proportion to some variable of interest (population, income).
Cartograms are popular visualizations for geo-referenced data that have been
used for over a century and that make it possible to gain insight into patterns
and trends in the world around us. Despite the popularity of cartograms and the
large number of cartogram types, there are few studies evaluating the
effectiveness of cartograms in conveying information. Based on a recent task
taxonomy for cartograms, we evaluate four major different types of cartograms:
contiguous, non-contiguous, rectangular, and Dorling cartograms. Specifically,
we evaluate the effectiveness of these cartograms by quantitative performance
analysis, as well as by subjective preferences. We analyze the results of our
study in the context of some prevailing assumptions in the literature of
cartography and cognitive science. Finally, we make recommendations for the use
of different types of cartograms for different tasks and settings
Bayesian model averaging over tree-based dependence structures for multivariate extremes
Describing the complex dependence structure of extreme phenomena is
particularly challenging. To tackle this issue we develop a novel statistical
algorithm that describes extremal dependence taking advantage of the inherent
hierarchical dependence structure of the max-stable nested logistic
distribution and that identifies possible clusters of extreme variables using
reversible jump Markov chain Monte Carlo techniques. Parsimonious
representations are achieved when clusters of extreme variables are found to be
completely independent. Moreover, we significantly decrease the computational
complexity of full likelihood inference by deriving a recursive formula for the
nested logistic model likelihood. The algorithm performance is verified through
extensive simulation experiments which also compare different likelihood
procedures. The new methodology is used to investigate the dependence
relationships between extreme concentration of multiple pollutants in
California and how these pollutants are related to extreme weather conditions.
Overall, we show that our approach allows for the representation of complex
extremal dependence structures and has valid applications in multivariate data
analysis, such as air pollution monitoring, where it can guide policymaking
Continuous-variable quantum key distribution in non-Markovian channels
We address continuous-variable quantum key distribution (QKD) in non-Markovian lossy channels and show how the non-Markovian features may be exploited to enhance security and/or to detect the presence and the position of an eavesdropper along the transmission line. In particular, we suggest a coherent-state QKD protocol which is secure against Gaussian individual attacks based on optimal 1 ->2 asymmetric cloning machines for arbitrarily low values of the overall transmission line. The scheme relies on specific non-Markovian properties, and cannot be implemented in ordinary Markovian channels characterized by uniform losses. Our results give a clear indication of the potential impact of non-Markovian effects in QKD
Dynamical paths and universality in continuous variables open systems
We address the dynamics of quantum correlations in continuous variable open
systems and analyze the evolution of bipartite Gaussian states in independent
noisy channels. In particular, upon introducing the notion of dynamical path
through a suitable parametrization for symmetric states, we focus attention on
phenomena that are common to Markovian and non-Markovian Gaussian maps under
the assumptions of weak coupling and secular approximation. We found that the
dynamical paths in the parameter space are universal, that is they do depend
only on the initial state and on the effective temperature of the environment,
with non Markovianity that manifests itself in the velocity of running over a
given path. This phenomenon allows one to map non-Markovian processes onto
Markovian ones and it may reduce the number of parameters needed to study a
dynamical process, e.g. it may be exploited to build constants of motions valid
for both Markovian and non-Markovian maps. Universality is also observed in the
value of Gaussian discord at the separability threshold, which itself is a
function of the sole initial conditions in the limit of high temperature. We
also prove the existence of excluded regions in the parameter space, i.e. of
sets of states which cannot be linked by any Gaussian dynamical map.Comment: 7 pages, 2 figures, improved pictures and forma
Dynamical decoupling efficiency versus quantum non-Markovianity
We investigate the relationship between non-Markovianity and the
effectiveness of a dynamical decoupling protocol for qubits undergoing pure
dephasing. We consider an exact model in which dephasing arises due to a
bosonic environment with a spectral density of the Ohmic class. This is
parametrised by an Ohmicity parameter by changing which we can model both
Markovian and non-Markovian environments. Interestingly, we find that
engineering a non-Markovian environment is detrimental to the efficiency of the
dynamical decoupling scheme, leading to a worse coherence preservation. We show
that each dynamical decoupling pulse reverses the flow of quantum information
and, on this basis, we investigate the connection between dynamical decoupling
efficiency and the reservoir spectral density. Finally, in the spirit of
reservoir engineering, we investigate the optimum system-reservoir parameters
for achieving maximum stationary coherences.Comment: 6 pages, 4 figure
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